give a closer look at the u-joints in the front drive axles, not just visibly, but also see if theres any play. another thing to look for is ball joints, upper and lower, front wheel bearings, and the front sway bar. does it only pop when its rolling? have you made sure that the hubs are unlocking/unlocked when you bring it out of 4wd? you do know that you need to back up about 5 feet or so after disengaging the 4wd so it will unlock the hubs? and last, does the front driveshaft still turn after you take it out of 4wd?